Is your feature request related to a problem? Please describe.
Right now, if you do shan install on a package that's already installed, shan will skip reinstalling it and just add it to your config. However, sometimes you want to update packages too. Shan should provide a way to update your packages.

Describe the solution you'd like
A way to do shan update <package-name> to update a package. shan update to update all packages. Maybe add a way to pin specific packages so that they don't get updated though since updates can sometimes introduce breaking changes.

Describe alternatives you've considered
You can always update through the actual package manager, but shan should be able to wrap around most of the basic functionality.